1. Field of the Invention
The present invention relates to process automation and, more particularly, to a field device for process automation.
2. Description of the Related Art
In field devices, a digital measurement, control or regulation signal is often generated by digital signal processing, and is then converted into a corresponding low-frequency analog voltage or current signal.
One example of this is a regulating or actuating device in which a signal processing device (e.g., a digital regulator) generates a digital control or regulation signal that is then converted, by an output stage, into an analog voltage or current signal for driving an actuator such as a valve.
Another example is a measuring transducer that is used to detect a physical variable, such as temperature, and to convert the variable into an output signal that is transmitted to a device, such as a control or regulating device. Here, a sensor that detects the physical variable provides an electrical raw signal that is generally analog. The output signal from the measuring transducer may be digital or analog, where the output signal is a current signal, usually in the range of 4 to 20 mA, in the latter case considered here. A higher-frequency digital signal in the kHz range can be superimposed on the analog current signal which, at up to approximately 100 Hz, is a low-frequency signal, according to the HART specification. In this case, the raw signal provided by the sensor is processed to form a digital measurement signal using a signal processing device, where the signal processing generally comprises analog signal processing steps, such as analog filtering and pre-amplification, as well as digital signal processing steps, such as zero point determination, scaling (i.e., determination of the measurement range) and signal conditioning using a specific transfer function. The digital measurement signal is then converted into the desired analog current signal using an output stage. Here, the digital measurement, control or regulation signal must be previously converted into an analog signal to generate the analog voltage or current signal. This may be effected, for example, by pulse width modulation (PWM) and downstream analog filtering, as is known from DE 10 2005 018 398 A1.
It is a known practice to perform signal processing in a redundant or diverse manner and to check the signal processing results for a match to detect errors during signal processing. It is also a known practice to “back-calculate” the result of a signal processing algorithm and to compare the back-calculated result with the input signal.
WO 03/060851 A1 discloses a measuring transducer in which an auxiliary signal is formed in a monitoring device from the raw signal provided by a sensor after passing through signal preprocessing, where the auxiliary signal is compared with the output signal, for example, a 4 to 20 mA current signal, from the measuring transducer to trigger an error message in the event of a difference that exceeds a predefined amount and to set the output signal in a safety-oriented manner. In order The raw signal is processed in a further microprocessor, which is provided in addition to the signal processing microprocessor, with the signal processing transfer function to form the auxiliary signal.
The conversion of the digital measurement signal generated by the signal processing device into the analog output signal is influenced by the dynamic response of the output stage, for example, a signal delay caused by the time constant of the output stage, which makes it difficult to compare signals and may result in undesirable error messages.